The invention relates to a method of determining the crash phases relevant to the triggering of a passive safety device in a vehicle.
Safety systems in motor vehicles include passive safety devices, such as air bags, belt tighteners, rollover bars and the like. In the event of a dangerous accident (in the following, also called a “crash”), particularly of a dangerous impact, those passive safety devices are triggered in order to protect the vehicle occupants as much as possible from injuries. To accomplish this, it is necessary to trigger the individual safety devices in each case at an optimal point in time. The prerequisite for doing so is to perfect the recognition of an accident with respect to the type of crash and the seriousness of the crash.
An algorithm provided for this purpose, which analyzes the output signals of at least one crash sensor, should be able to recognize or differentiate between, for example, the following characteristics of a crash:                a) Start of the crash;        b) Type of crash: Offset 100% to 25%;        c) Crash impact angle 30° to 90°; and        d) Crash impact speed.        
However, the crash algorithm not only has to be able to differentiate between greater number of crash events, it also has to be extremely stable. This means, on the one hand, that it has to be possible, by using the algorithm, to also recognize accidents (for example, the impact of a vehicle against a soft obstacle at a low speed), in which the safety devices should not be triggered (so-called no-fire crashes), or when only some of the existing safety devices should be triggered. On the other hand, it has to be possible to also unambiguously recognize operating conditions of the vehicle which generate a crash sensor signal course that differs only insignificantly with respect to the signal course of the crash sensor in an actual crash, for example, in the case of an extreme stressing of the vehicle, as occurs, for example, during fast driving along a gravel and/or pothole plagued stretch of roadway.
Another difficulty consists of the fact that the above-mentioned information concerning a crash has to be available at a very early point in time relative to the start of the crash. As a rule, a crash against a rigid obstacle at a high speed requires an extremely early firing point in time, usually of less than 10 ms. However, the amount of information available up to this point in time is very small, and as a rule not “solid” or reliable information. This applies at least when, for example, one or more acceleration sensors are used as crash sensors and a threshold value is used as a triggering criterion for the safety devices. As experience shows, no-fire crashes may also exceed this threshold value.
It is an object of the invention to provide a method of determining the crash phases relevant to the triggering of a passive safety device in a vehicle, which method is reliable and permits an unambiguous recognition of the above-mentioned crash characteristics at an early stage.
This object is achieved by providing a method of determining the crash phase relevant to the triggering of a passive safety device in a vehicle, characterized in that an acceleration signal (ax) is determined which is representative of the acceleration (ax(t)) acting in the longitudinal direction of the vehicle. With the start of the crash-caused deceleration of the vehicle, simultaneously two window integrals are started by which the acceleration signal (ax) is added or integrated over the defined time windows. The first time window is smaller than/equal to the minimal triggering time for triggering the safety device and the second time window is significantly larger than the first time window. The time period during which the window integrals have the same value and during which this value is greater than a threshold value is defined as a first crash phase.
The reliability of the information concerning the crash is achieved by integral formation. The above-mentioned no-fire cases, as well as the above-mentioned excess vehicle stressing situations, when a corresponding threshold value is defined, can be differentiated from an actual crash with a necessary triggering of the safety devices. As a result of the selection of the integration time of the first window integral according to the invention, the crash type also becomes determinable by the earliest required triggering of the safety device. It can be derived from the value of the window integral. Finally, the start of the crash can also be determined.
The start of the crash can be recognized through the use of a separate sensor system. Thus, precrash sensors are known which react to objects approaching the vehicle. In this case, a conclusion can be drawn from the approach speed that a crash is imminent. With respect to the latter, the invention offers the possibility of calculating the start of the crash back from the window integrals themselves. The start is equivalent to the start of the first crash phase 1. If the first (and thus also the second) window integral has a value which is above the threshold value, a calculation can take place back to the point in time at which the integral formation began. This point in time corresponds to the start of the first crash phase 1.
The second crash phase 2 following first crash phase 1 can also be determined by use of the window integrals. The second crash phase 2 has certain characteristics. Its start depends on the start and on the duration of the first crash phase 1. Since these characteristics are certain, the start of the second crash phase 2 is also defined. This point in time is almost identical for all crash types because the former characteristics of the first crash phase 1 are also independent of the type of crash.
The second crash phase 2 exists when the second window integral has a higher value than the first window integral. This information is obtained on the basis of the different computation of the two integrals and, with respect to the processing, becomes noticeable in that the two window integrals have an integration start which increasingly differs with an increasing crash duration. That of the first window integral is increasingly displaced and is in each case set back by the window length with respect to the actual point in time, while the second window integral retains its integration start. The latter is characterized by the start of the first crash phase 1.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.